Looking at a typical site, a reasonable user might want to accept one (or perhaps a couple) of many dozens of cookies a site attempts to set. Choosing it manually per site per cookie is difficult but perhaps theoretically possible, however even that still requires cooperation from the site to honestly identify that this one is the cookie which is functionally required, and these fifty are for ad tracking, and ensuring that cooperation still requires legal means and can't be done with purely technical ones.

Furthermore, there is the important distinction about multiple uses of the same data. There are uniquely identifiable cookies that are functionally required for one purpose but the site may want to use it for other purposes as well (e.g. share that data with heir "trusted partners" for targeted advertising) for which user may reasonably want to refuse permission, so a browser accepting a cookie doesn't imply such permission and something extra is required.